This is a double-blind Phase 2a study to test the safety and efficacy of an investigational HIV drug, amdoxovir (300 mg or 500 mg twice daily) compared with tenofovir DF 300 mg once daily in HIV-1 infected antiretroviral therapy-experienced subjects who are currently failing antiretroviral therapy. There are three treatment groups (N=45). Subjects will be randomized to receive either amdoxovir 300 mg twice daily (n=15) or amdoxovir 500 mg twice daily (n=15) or tenofovir DF 300 mg once daily (n=15); each in combination with zidovudine 300 mg twice daily.